The present invention relates in general to fastening elements, and, more particularly, to elements used to control the amount of torque applied to fastening elements.
In many applications of threaded fastening devices, the value of torque applied thereto is critical. The application of too little torque results in accidental loosening of the fastening device, while excessive tightening may cause damage to the fastening device or adjacent parts. In most applications, it is impossible to visibly determine whether or not the appropriate torque has been applied to an in-place threaded fastening.
As the off-the-shelf fastener is not, itself, torque limiting, the difficulty in visually determining whether the proper amount of torque has been applied to a fastener has engendered disclosure of devices which are useful in controlling the amount of torque applied to such fasteners. Examples of such devices are torque wrenches and torque measuring devices. While useful, these tools and systems are expensive and often cumbersome. Additionally, the use of torque wrenches depends upon individual initiative and, therefore, it is difficult to assure the torque wrench has been properly used, if at all.
Other torque controlling devices known to the inventor include torque control means integral with the fastening device (see U.S. Pat. No. 4,037,515, issued to D. A. Kesselman on July 26, 1977). These systems usually provide for a severable, weakened extension of the fastening device, itself, which is designed to fail at a predetermined torque value. While these systems are somewhat effective, they are expensive, requiring use of the expensive materials from which the fastening member itself is constructed, and additionally, requiring special machining or forming operations. Also, because of the special designs required, this class of system precludes the use of standard, off-the-shelf fastening means.
As used herein, the term "off-the-shelf" is taken to mean those items commonly and presently available. Such items as screws, bolts, and the like, which are readily purchased in normal commercial distribution outlets, such as hardware stores, drugstores, and the like, are "off-the-shelf" items within the meaning attached to that term in the present disclosure, as opposed to items which must be purchased in specialty stores, or ordered specially.
Other devices known to the inventor include those devices disclosed in U.S. Pat. Nos. 3,460,428, 3,929,054 and 4,068,555. However, all of the devices disclosed in these just-mentioned patents require special torque limiting fasteners to function as a torque limiter. These special fasteners serve as the fastening element, and thus replace the off-the-shelf fastener. Thus, these devices are not themselves typical off-the-shelf items, and are not amenable for use with off-the-shelf hardware, and therefore must be specially purchased and used. The problems and drawbacks to such limitations include cost and availability, as well as the difficulties caused the person using the device.
A clamp tightener is disclosed in U.S. Pat. No. 2,503,189, issued to E. Biba, Jr. on Apr. 4, 1950, and includes a spring clamped over a non-circular post of a fastener. The spring is accommodated within a chamber of a cap member, and when torque exceeds a predetermined value, the spring simply overrides the edges of the non-circular post. While such a device is somewhat effective, it has several drawbacks, such as, for example, the non-uniformity caused by relying on the holding force exerted by a spring under various conditions, the possibility of exerting too little torque due to a loosened condition for the spring, and the like. The Biba device may be satisfactory for hose clamps and other finger tightened type devices, but may not be reliable to achieve accurate and high enough torque values or otherwise be suitable for applications involving wrenches, screwdrivers, or other such torquing elements.
Accordingly, there is need for a torque limiter which can be used in conjunction with off-the-shelf fasteners which are not themselves torque limiting for limiting the amount of torque applied to such off-the-shelf fasteners.